This invention relates to an improved apparatus which can be generally utilized for plasma processing, or more specifically for either plasma etching and cleaning or for plasma deposition. In particular, this invention relates to an apparatus which provides an improved uniformity of etch rate and an improved uniformity of deposition rate across the entire surface of the work piece.
The plasma chemistry technique provides a new approach to industrial materials processing in a variety of applications. It eliminates the need for high temperatures or wet chemicals in the etching or other preparation of small electronic, optical, and mechanical parts. In the plasma chemistry technique, the material or specimen is placed in or on a sample holder which is loaded into the reaction chamber of the apparatus. The chamber is evacuated to a mild vacuum by a mechanical vacuum pump and the reaction gases are drawn into the chamber to surround the sample. Radio frequency power is applied to the chamber to excite the reaction gas molecules and to change some of them to other species, such as atoms, radicals, ions, and free electrons. This gaseous plasma is highly reactive and it causes a low temperature reaction of the species in the gas or on the surface of the specimen. The waste products of the reaction are carried away in the gas stream, leaving the base specimen material for further processing. By using different gases, highly selective reactions with a sample can be obtained. In order to etch the sample, for example, the gas can be selected from a number of either oxidizing or reducing gases. If it is desired, instead, to deposit a film upon the specimen, a reaction gas could be selected which is a source of the film material. For example, to deposit a film of silicon nitride, the reaction gas could be a mixture of silane, ammonia, and nitrogen. To deposit silicon, the reaction gas could be silane, a halosilane, or a silicon halide.
Among the plasma reactors known in the prior art are the so-called radial flow reactors. In these reactors, which are specifically designed for plasma deposition, an annular electrode forms the support for the material specimens which are disposed around the circumference on a surface of the support. Gas flow is established over the outer edge of the support and flows in a laminar manner radially over the specimen and is exhausted through an exhaust port located in the center of the support. As the gas flows in this radial manner a glow discharge is effected by an RF field established between the support and the second parallel electrode positioned above the specimens. The most serious deficiency of this apparatus is the lack of uniformity of the thickness of the deposit along a radius because of source gas depletion effects.
Cylindrical reactors are also known and used in the prior art. As the gas passes through the reactor, it is ionized by the high frequency electromagnetic field established around the reactor. These reactors are principally used for etching or cleaning, but could also be used for deposition by a proper selection of the reactant gas. Like the aforementioned radial flow reactor, the cylindrical reactor also suffers from non-uniformities. Again, the non-uniformity is due in part to source gas depletion as the reaction gas flows through the reactor.
In general, because the plasma consists of a neutral cloud of electrically charged particles, there is a finite probability that two charged particles can recombine to form a neutral and unreactive molecule. This effect, of course, increases with both time and distance traveled by the ionized species. Thus the density of the reactant species will vary as a function of the distance away from the inlet orifice through which the reactant gas enters the chamber. This results in a uniformity problem which exists with either of the aforementioned reactors. This problem is more critical with some gases than with others because of different reactive lifetimes. For gases having short reactive lifetimes, it is commonplace to use high vacuum pumping rates to move the gases quickly through the reactor and thereby minimize depletion effects. The high pumping rates require large, expensive pumps and are wasteful of reaction gases as quantities of unreacted gases are expelled from the reactor.
In certain applications, such as the fabrication of semiconductor devices, the coating of optical lenses, or the fabrication of precision mechanical parts, extreme uniformity is required, both across a single work piece and from one work piece to another. Accordingly, a need has existed for a plasma reactor design which affords the high precision and uniformity necessary for these and similar applications.